1. Field of the Invention
The invention relates to a silicone mixture crosslinkable under the influence of light and having high initial strength, and to a process for producing a silicone fiber from the silicone mixture.
2. Description of the Related Art
For the ultrafast transport of electronic data over large distances, optical fibers are increasingly being used. The demands on optical purity are very high, since every single particle leads to scattering losses which, accumulated over the transport distance, lead to great losses and possibly incorrect data. For the production of the correspondingly required high-transparency optical fibers from silicone, there has to date been no suitable extrusion process, since the transparent, filler-free, silicone resin-reinforced silicone formulations do not have sufficient initial strength (green strength) to enable a continuous production process with conventional extrusion technology.
Tubes or fibers made from silicone are produced by extrusion. This involves extruding the crosslinkable formulations through a shaping die in a continuous process and then vulcanizing in a heating tunnel at temperatures above 200° C. In order that the pipe does not break off and retains its geometry at the high crosslinking temperatures, silicone formulations with high green strength are required for this purpose. The high green strength is achieved by means of correspondingly high-viscosity HTV silicone polymers having chain lengths of typically 5000 with simultaneous addition of finely divided silica (HDK® silica). The HDK silica has additional thickening action and is necessarily required in order to obtain sufficient green strength for shape retention and thermal vulcanization.
WO 2009/027133 describes light-activatable silicone mixtures for extrusion of moldings. These silicone mixtures contain ultrahigh-viscosity silicone polymers with chain lengths greater than 3000, in order to achieve, in combination with the HDK filler, sufficient green strength with a high Mooney viscosity of greater than 10. The formulations described cannot be used to produce high-transparency silicone moldings, since the filler scattering losses are too high.